*2.6. Evaluation of Physical, Mechanical and Surface Properties of Ceramic Materials and Coatings*

The hardness of sintered ceramic compositions based on SiAlON was determined at a load of 2 kg by the Vickers pyramid indentation method on a QnessQ10A universal microhardness tester manufactured (Qness GmbH, Mammelzen, Germany).

The sintered samples' density was estimated using the hydrostatic weighing technique based on Archimedes' law comparing the mass of ceramic samples obtained in air and liquid under normal conditions. The measurements were carried out in distilled water on a GR-300 high-precision analytical balance (A&D Company, Tokyo, Japan).

The samples' strength characteristics were measured by 3-point bending tests at room temperature on an AutoGraph AG-X universal testing machine (Shimadzu, Kyoto, Japan). The loading rate during the tests was 0.5 mm/min, and the maximum displacement was 40 mm. Fracture toughness was also assessed using AutoGraph equipment using a similar loading pattern. A one-sided notch perpendicular to its longitudinal axis was made on ceramic samples' surface with a diamond disk. The sample was installed with a notch downwards and loaded.

The morphology and structure of sintered ceramic specimens and DLC coatings were studied using a VEGA 3 LMH scanning electron microscope (SEM) (Tescan, Brno, Czech Republic) equipped with an Oxford Instruments INCA Energy energy-dispersive X-ray spectroscopy (EDX) system. The sample atoms under study were excited and emitted Xrays characteristic of each chemical element with an electron beam's assistance in the EDX process. When studying the energy spectrum of the indicated radiation using a specialized program, data on the samples' qualitative and quantitative elemental composition were obtained. The program identifies an element and even reveals hidden elements in the sample using the point analysis mode. Transmission electron microscopy (TEM) and selected area diffraction (SAED) studies of the surface layer of ceramic samples with DLC coatings were carried out on JEM-2100F equipment manufactured by JEOL (Tokyo, Japan). The samples were prepared according to the standard probe techniques using Opal 410, Jade 700, and Saphir 300 sample equipment (ATM, Haan, The Netherlands). An epoxy resin with quartz sand was used as a filler for SEM samples. The samples were coated with gold by Quorum 150T ES (Laughton, UK). An instrument controlled the thickness of the coating of less than ~30 μm. TEM samples were polished, cut off 100 nm by an ARTOS 3D Ultramicrotome (Leica, Wetzlar, Germany), and sputtered with gold.

An experimental technique and a special stand, described and tested by the authors in [25], were used to assess the effect of coatings on the resistance of sintered ceramic specimens to fracturing under the action of external loads. The stand was a device in the form of a massive parallelepiped with a groove and step. Sintered ceramic specimens in the form of 20.0 mm diameter and 3.0 mm thickness disks were installed on the step with a small gap. During the experiment, a force was applied to the punch, rigidly fixed on the INSTRON universal testing machine, until the plate broke. A mandatory requirement during the test was to ensure strict reproducibility of the test process, particularly the identity for the entire group of samples of the place of application of force from the punch to the ceramic sample. In the experiment's course, the diagram "force – the punch movement along the z-axis" was recorded, and the critical force at which the destruction occurred was recorded.

The surface microroughness of the sintered ceramic samples before and after DLC coatings' deposition was evaluated on a DektakXT stylus profilometer (Bruker, Billerica, MA, USA).
